Programmable time varying control system and method

ABSTRACT

An electronic system and method are disclosed for controlling a measured temperature during a sequence of time intervals, in accordance with a sequence of selected reference temperatures each corresponding to one of the intervals. The values of the reference temperatures are programmable and changeable by the user. The system and method can be used to establish thermostatic control in conformity with a referenced temperature that varies throughout the day, for example, hour by hour. The system also provides for using different sequences of reference temperatures on different days.

RELATED PATENTS AND APPLICATIONS

This application claims priority under the provisions of 35 USC 120, onthe basis of the co-pending U.S. patent application Ser. No. 135,531filed Mar. 31, 1980, and which in turn depends from co-pending U.S.patent application Ser. No. 873,093 filed Jan. 30, 1978 and now U.S.Pat. No. 4,200,910, and which in turn depends from co-pending U.S.patent application Ser. No. 774,393 filed Mar. 4, 1977 and now U.S. Pat.No. 4,071,745.

BACKGROUND OF INVENTION

This invention relates to a system and method for programmable timevarying control and, more particularly, to a thermostatic system andmethod in which the reference temperature can be made to vary hour byhour in a programmed way.

In a control system operating against a significant load, considerableenergy can be required to hold the controlled variable at or near areference value. This happens in a thermostatic system when theuncontrolled value of temperature differs substantially from the desired(reference) temperature. It is a very effective means of reducing energyconsumption under such circumstances to identify periods when lesscontrol is required and to provide for reducing control during thoseperiods. For example, in the thermostatic control of cooling a home, itis effective to raise the thermostat reference temperature during theearly hours of the day before the external temperature reaches itsmaximum, during sleep periods and for periods when the occupants aregoing to be away. This can, of course, be done manually. However, in amanual system, it can be overlooked or forgotten and cannot be done astimely or accurately as with an automatic system. Another drawback of amanual system is its inconvenience, for example, if the referencetemperature is raised when the occupants leave for some time, when theyreturn, the temperature in the home has equilibrated at an uncomfortablyhigh value and may take considerable time to reach a comfortable range.The present invention provides a system and method wherein the referencetemperature is made to vary during the day, assuming a sequence ofprogrammable values. This allows heating or cooling to be automaticallyinitiated so as to reach a new reference temperature by a particulartime, for example, when the occupants of a home are to return.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a system forcontrolling a measured temperature during a sequence of time intervals,in conformity with a sequence of selected reference temperatures, eachcorresponding to one of the intervals. The system has means for storingdigital numbers, each of which represents a value of one of thereference temperatures. It also has means for storing a digital numberrepresentative of a value of hysteresis. Further there are means, whichcan be connected to a reference frequency source, to repeatedly generatea digital number that is representative, at each generation, of thepresent time of day. An updating means repeatedly determines, from thetime of day number, when one of the time intervals embraces the presenttime, and identifies the reference temperature corresponding to theembracing interval. The updating means stores, as a current referencetemperature value, a digital number representing the value of theidentified reference temperature. The updating means includes means fordetermining that as soon as the present time exceeds the last timeinterval in the sequence, the present time is treated as being in thefirst interval in the sequence. The system has means which can beconnected to a sensor of the measured temperature for generating anelectrical signal representation of the measured temperature. Acomparison means, responsive to the signal representation, to thecurrent reference temperature number and to the hysteresis number,repeatedly generates control signals suitable for heating and/or coolingsystems affecting the measured temperature. The hysteresis valueestablishes how far the measured temperature may deviate from thereference temperature, before heating or cooling is initiated.

In a preferred embodiment of the invention, there is provided means forstoring a digital number representative of an anticipator value. In theembodiment, the comparison means includes apparatus for generatingcontrol signals capable of activating the heating and cooling system,when the measured temperature deviates from the current referencetemperature by more than the hysteresis value. The comparison means alsoincludes apparatus for generating control signals capable ofdeactivating the heating and/or cooling system, when the measuredtemperature is short of the current reference temperature value by theamount of the anticipator value. After the heating and/or cooling systemis deactivated, residual heat or cold in the elements thereof, cause themeasured temperature to reach the reference temperature value.

Also, in a preferred embodiment, means are provided allowing the user toenter the reference temperature numbers, so that he may select andchange the reference temperature profile according to his ownrequirements. Additionally, a preferred embodiment includes means forthe user to enter a digital number representing a reference temperatureadjustment, wherein the comparison means includes apparatus forperforming the generating of control signals, with the current referencetemperature value modified by the value of the adjustment number. Thisallows changing the temperature of the controlled environment for anindefinite period, without affecting the stored reference temperatureprofile.

Additionally, a preferred embodiment of the system includes means forsimultaneously utilizing a higher, cooling reference temperature and alower, heating reference temperature. The system can then generatesignals capable of activating a cooling system, when the measuredtemperature is above the higher reference temperature and activating aheating system, when the measured temperature is below the lowerreference temperature.

An alternate embodiment of the invention is capable of multi-stageoperation. That is, the comparison means generates control signalscapable of commanding the heating and/or cooling system to heat or coolat more than one rate. The rate commanded is dependent on the amount ofthe deviation of the measured temperature from the current referencetemperature value. This allows the heating and/or cooling system toaffect the measured temperature faster, when the deviation from thereference temperature value is large.

Yet another embodiment of the invention is a system for controlling ameasured temperature in each of a plurality of zones, during a sequenceof time intervals. The temperature is controlled in accordance with asequence, for each zone, of reference temperature values, eachcorresponding to one of the time intervals. Included are means forgenerating, for each zone, control signals for apparatus affecting thetemperature of air delivered to the zone.

In an improved embodiment of the invention, the user can select onesequence of reference temperatures to be used on days when he is at workand a different sequence to be used, when he is home from work.

The present invention also provides a method for the control of ameasured temperature during a sequence of time intervals, in accordancewith a sequence of selected reference temperatures, each correspondingto one of the intervals. The temperatures, each corresponding to one ofthe intervals. The method includes storing, in electronically readableform, a plurality of digital numbers, each representing a value of oneof the reference temperatures. Also stored in electronically readableform is a digital number representative of a value of hysteresis. Themethod further includes repeatedly generating, in electronic form, adigital number representing the present time of day at each generation.By an electronic comparison including the time of day number, it isrepeatedly determined when one of said intervals embraces the presenttime. This determination includes deciding that as soon as the presenttime exceeds the last interval in the sequence of intervals, said timebecomes embraced by the first interval in the sequence. Anidentification is made of the reference temperature corresponding to theembracing interval. A digital number representing the value pf theidentified reference temperature is stored, in electronically readableform, as a current reference temperature value. An electrical signalrepresentation of the measured temperature is generated; then there iselectronically generated a digital number representing a function of thecurrent reference temperature and the hysteresis number. The functionnumber and the representation of measured temperature are electronicallycompared and, based on the results signals are generated suitable tocontrol means affecting the measured temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a thermostatic control system according tothe invention.

FIG. 2 is a set of graphs showing possible profiles of referencetemperatures over a twenty-four hour day.

FIG. 3 is a collection of graphs showing, in schematic fashion, the timevariation of measured temperature under control of a system inaccordance with the invention.

FIG. 4 is a graph showing the time variation of measured temperature ina system capable of multiple stage operation according to the invention.

FIG. 5 is a partial block diagram of a system capable of multiple stageoperation in accordance with the invention.

FIG. 6 is a schematic diagram of a multiple zone temperature controlsystem embodying the invention.

FIG. 7 is a partial block diagram of a system for performing multiplezone temperature control in accordance with the invention.

FIG. 8 is a block diagram of alternate means for digitizing a measuredtemperature signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention can be implemented with various electronicsystems. One approach is the use of logic circuit chips or modulesinterconnected to perform the functions required. The approach to bedescribed herein employs a programmed microprocessor. The descriptionthat follows sets forth functions that must be performed by units of themicroprocessor, in the operation of the system according to theinvention. For a particular function to be performed, the microprocessormust be progammed to step through a sequence of instructions thatcommand the described function. In some cases, the statement of functionis practically equivalent to specifying a sequence of instructions. Forexample, if two numbers are to be compared, this is accomplished on somecomputers by the conventional code sequence of (1) loading a registerwith the contents of a memory location containing one of the numbers,and (2) comparing the number in the register with the contents of amemory location containing the other number. In other cases, moreinstructions will be required, but it will be understood how to design aprogram providing the described functions, in the context ofconventional microprocessor practice.

FIG. 1 shows a thermostatic system according to the invention, indicatedgenerally by the reference nemeral 10. The microprocessor in system 10includes an arithmetic and control unit 11, a random access memory 13,and a read-only memory 12 for the storage of constants and the operatingprogram. Data entry switches 14 allow the selection of digits of anumber to be entered into the microprocessor. Function switches 15 areused to designate the nature of the date being entered on the data entryswitches 14. Mode switches 16 permit selection of an operating mode forsystem 10. A display unit 18 provides alphanumeric and on-off displaysfor a number of the variables and parameters of system 10. Unit 18 canbe of the light emitting diode type, a liquid crystal display or agas-discharge type electronic display.

A transducer 20 produces an electrical signal proportional to thepresent temperature in a region where the temperature is to becontrolled. A signal conditioning circuit 22 contains an amplifierhaving suitable response for enhancing the quality of the transducersignal passed to the microprocessor. The gain of circuit 22 providesscaling appropriate to the next circuit, an analog-to-digital converter24, which provides to unit 11 a digital representation of thetemperature sensed by transducer 20. An output circuit 26 is responsiveto logic states in unit 11 to produce signals capable of providingsingle or multiple stage control for a cooling system 28 and a heatingsystem 30 which affect the region where transducer 20 is located. Outputcircuit 26 can have, for example, analog, digital or discrete (on/off)type outputs utilizing amplifiers, linedrivers, triacs or relays thatinterface with cooling system 28 or heating system 30.

TYPICAL USE

Thermostatic system 10 accepts a sequence of reference temperaturevalues, each corresponding to one of a sequence of time intervals.Preferably, there are 24 possible time intervals so that referencetemperatures may be entered for each hour of the day. Then, during thethour of the day, temperature is controlled by system 10 in accordancewith the reference temperature for that hour.

FIG. 2 shows several possible sequences of reference temperatures, RTfor cooling and heating. The FIGURES show how the reference temperature,RT settings vary with the time of the day. In the FIGURES, one timeinterval has been assumed to be equal to one hour. The measuredtemperature response to these reference temperature values is a functionof the heating/cooling capacities of the systems, thermal timeconstants, heat loads, external temperature/weather conditions, etc.

FIG. 2A depicts a cooling case where the user is not home during theday. The temperature is kept at 79° during the low activity sleepperiod. At 6:00 A.M., the temperature is lowered to 77° and then to 76°at 7:00 A.M. At 8:00 A.M., it is raised to 80° as the user leaves forwork. The temperature stays at this value until 5:00 P.M. when it is setto 77° and then lowered to 76° as the user arrives home from work. From6:00 P.M. to 10:00 P.M., it is kept at 76°. At 10:00 P.M., it is raisedto 77°, and then further raised to 79° at 11:00 P.M., as the sleepperiod begins.

FIG. 2B depicts a cooling case where the user is home during the day. Asbefore, during the sleep period, the temperature is set at 79°. At 6:00A.M., it is lowered to 77°, where it remains until 10:00 A.M. At 10:00A.M., it is lowered to 76° until 1:00 P.M., when it is then lowered to75° as the external temperature continues to increase towards themaximum for the day. As the external temperature begins to cool, thetemperature is increased to 76° at 7:00 P.M., where it remains until10:00 P.M. From 10:00 P.M. to 12:00 P.M., it is set at 77°. Thetemperature is set at 79° at 12:00 P.M., as the sleep period begins, andit remains there until 6:00 A.M.

FIG. 2C depicts a heating case where the user is not home during theday. The temperature is set for 66° during the sleep period. At 6:00A.M., it is raised to 69°, where it remains until 8:00 A.M. At 8:00A.M., as the user leaves for work, the temperature is lowered to 66°.From 8:00 A.M. until 5:00 P.M., it remains at 66°. At 5:00 P.M., thetemperature is raised to 70° as the user returns home from work near6:00 P.M. The temperature is left at 70° until 10:00 P.M. From 10:00P.M. to 11:00 P.M., it is lowered to 69° and at 11:00 P.M., it islowered to 66° as the sleep period begins.

FIG. 2D depicts a heating case where the user is home during the day. Asin the above case, the temperature is set for 66° during the sleepperiod until 6:00 A.M. At this time, it is raised to 68°. At 7:00 A.M.,it is raised again to 70°, where it remains until 11:00 A.M. At 11:00A.M., it is lowered to 69° as the external temperature increases. At6:00 P.M., as the external temperature begins to decrease, thetemperature is raised to 70°, where it remains until 10:00 P.M. From10:00 P.M. to 11:00 P.M., it is set back to 69°. At 11:00 P.M., thetemperature is lowered to 66° as the sleep period begins.

OPERATION

Provision is made in the system 10 for the user to conveniently enterthe values of reference temperatures corresponding to the variousintervals of the day, making use of display 18. Ordinarily, display 18shows the current values of time (including A.M. or P.M.), referencetemperature and measured temperature. There are two methods ofdesignating the particular one of the time intervals which is to haveassociated with it the reference temperature value being entered.According to the first method, a number designating the interval is setin data entry switches 14. One of the function switches 15 is activated,causing unit 11 to read the switches 14 as an interval designatingnumber and to fetch for display the reference temperature stored inmemory 13 for that interval. The reference temperature valuecorrsponding to the displayed time interval can then be initially chosenor changed by entering a number on data entry switches 14 and activatinga particular one of the function switches 15. Unit 11 then decodesswitches 15, and determines therefrom that the number from switches 14is to be entered as a reference temperature value. Unit 11 stores thenumber in the memory 13 location from which the value currently beingdisplayed was taken.

According to the second method, activation of one of the functionswitches 15 initiates display of the values of time and referencetemperature next in sequence from those currently being displayed. Thenthe data entry switches 14 can be used, as described above, to store anew reference temperature value in the place of the one displayed. Thus,beginning with the current time or an interval chosen by the firstmethod, a new value can be entered for the reference temperature, andthe function switch can then be activated to display the time andtemperature for the next interval in sequence. Then, each succeedingreference temperature may be examined and changed, if desired, bystepping from one interval to the next through the sequence.

Data entry switches 14 are used to enter a number for the durationassociated with each of the time intervals. One of the function switches15 commands unit 11 to store the number in memory 13 as the value of theinterval duration.

In order for the system 10 to keep time, an initial value is entered bymeans of the data entry switches 14 and function switches 15, and isstored by unit 11 in memory 13. Thereafter, circuits operating from astable time base, such as the 60 Hz line frequency or a crystalcontrolled oscillator, provide periodic clock signals to unit 11. Inresponse to the clock signals, unit 11 updates the value stored in thetime location in memory 13. It is this updated time which is shown ondisplay 18 as the current time, including an A.M.-P.M. indication.

The basic operation of system 10 is a cyclical process in which, duringeach interval of the day, the temperature measured by transducer 20 isrepeatedly compared with a stored reference temperature to generateappropriate control signals for the cooling and heating systems. Therecan be considerable variety in the detailed steps, by which theobjectives are achieved using a microprocessor. The following is onepossible sequence of suitable steps.

Periodically, unit 11 reads an output register of analog-to-digitalconversion unit 24 for the digital number representing the temperaturemeasured by transducer 20. Shortly before or after doing this, unit 11fetches from memory 13 the current updated value of time and a digitalnumber representing the time at which the current control interval ends.These two numbers or times are compared to detect when the time passesout of one control interval and into the next. If the comparison showsthat the present time is embraced by a new control interval, unit 11adds the interval duration to the number corresponding to the end of theold interval, and the new end time is stored in memory 13. In addition,the reference temperature corresponding to the new interval would beretrieved from the sequence of reference temperatures in memory 13 andstored in a different location allocated as the reference temperaturevalue currently used for control.

Precisely what happens next depends upon the mode of operation selectedfor system 10. This will be described in more detail below. Theselection of the operating mode is made through activation of modeswitches 16 by the user of system 10. Logic unit 11 reads the switchselection and stores a bit indicating the selected mode in memory 13. Atthose points in the control sequence when it is necessary to know theselected mode, logic unit 11 retrieves the stored bit from memory 13 androutes subsequent program steps accordingly.

Two parameters which are important to further processing are thehysteresis values, H, and the anticipator values, A. Each of these isentered by the user with data entry switches 14 and a function switch 15corresponding to the particular parameter. Unit 11 stores the numbersfrom switches 14 in the memory 13 location allocated to A or H, asappropriate. Alternatively, the values can be programmed in read onlymemory 12.

In the process of making a control decision, unit 11 determines theselected operating mode and fetches from memory 13 applicable values ofthe current reference temperatures, hysteresis and anticipators. Fromthe digital numbers corresponding to these quantities, unit 11 computesthe value of a function which depends upon the selected operating mode.Unit 11 then retrieves the digital number corresponding to the presentmeasured temperature from unit 13 and compares this to the functionalvalue just computed. According to control rules described below, unit 11derives from the results of this comparison a digital code to present tooutput circuit 26 to dictate relay openings and closings by circuit 26.

The preceding cycle of operations for producing control outputs based ona comparison of measured temperature with a function of a time varyingreference temperature is repeated at an interval selected to produceeffective temperature control, considering the thermal time constantsinvolved. For example, the period of repetition for the above steps canbe ten seconds or less.

The operations are also cyclical on a daily basis. The microprocessor isprogrammed such that when the present time increases beyond the end ofthe last control interval in the sequence, it is treated as beingembraced by the first interval in the sequence. Thus, if there are 24one-hour intervals, when the time exceeds the twenty-fourth hourlyinterval, it is considered to re-enter the first hourly interval.

OPERATING MODES

FIG. 3 illustrates operation of system 10 in four different operatingmodes. Each part of the FIGURE shows in a stylized way the variation oftime of the measured temperature MT at transducer 20. All the variationsshown are within the confines of a single time interval (e.g., an hour)in the sequence of control intervals. Those portions of the MT shown insolid line indicate periods in which heating or cooling is occurring.Where the graph is shown in dashed line, the temperature of thecontrolled space is drifting toward an unheated or uncooled state. Inthe various sections of FIG. 3, H is the value of hysteresis and A isthe anticipator value. The parameter RT is the current value of thereference temperature. The quantities RTH and RTL are current values ofa higher reference temperature and a lower reference temperature,respectively.

FIG. 3A illustrates system 10 operating in the Cooling mode. Initially,that is, when system 10 is first switched into this mode, if themeasured temperature MT is greater than RT+A, system 10 initiatescooling, driving the measured temperature down. Any time the measuredtemperature decreases to the level of RT+A, system 10 generates a codeto terminate cooling. The measured temperature continues to fall belowRT+A, however, because there is a certain amount of residual coldness insystem 28 after it is shut off. If the anticipator value A is wellchosen, this residual coldness will drive the measured temperature downto the reference value RT before the residual cold is dissipated. As thecooling system 28 finally ceases cooling, the measured temperaturevalue, near RT, begins to rise. Thereafter, whenever the temperaturerises to the value RT+H, the cooling system 28 is turned on and remainson until the measured temperature decreases to RT+A.

Thus, selection of value of the hysteresis H determines the acceptabledeviation in measured temperature from the time that the cooling systemturns off until it is turned on again. The value of A is preferablyselected just large enough so that residual cold in the cooling systemwill carry the measured temperature no further down than the referencetemperature. Typical values of H are 1-1.5° F. The value of a depends,of course, on the heating and cooling system used: typical values are0.5-0.8°F.

Each of the other operating modes of system 10 embodies concepts similarto those encountered in the cooling mode. The Heating mode is shown inthe FIG. 3B. Initially, the system 10 initiates heating when MT is lessthan RT-A. Thereafter, heating is begun whenever MT is less than orequal to RT-H. System 10 signals heating system 30 to cut off when MT isgreater than or equal to RT-A. After the heating system 30 is turnedoff, residual heat in system 30 carries the measured temperature on upto the value of the referenced temperature RT. Once the residual heat isdissipated, MT begins to fall, continuing until heating is again turnedon when MT reaches RT-H. The hysteresis and anticipator values enteredfor use in this mode can be different from those of the Cooling mode.

The Automatic mode, illustrated in FIGS. 3C and 3D, combines operationof the Cooling and Heating modes. In this mode, system 10 holds themeasured temperature within a programmable temperature range. The upperend of the temperature range is a higher reference temperature valueRTH; the lower end is a lower reference temperature value RTL. Thus, theAutomatic Mode requires, for twenty-four time intervals, a total offorty-eight reference temperature values. In simpler versions of system10, RTH is the same value of RT used for the Cooling mode, and RTL isthe RT for the Heating mode. In more complicated versions, totallydifferent sets of threshold values may be entered for each mode. Afterthe Automatic mode has been selected, the initial turn on of coolingoccurs if the measured temperature is greater than RTH+A. Thereafter, asparticularly illustrated in FIG. 3C, system 10 signals the coolingsystem 28 to turn off when MT becomes less than or equal to RTH+A, andit is turned back on if the measured temperature becomes greater than orequal to RTH+H. If conditions cause the measured temperature to enterthe region between the high reference temperature value RTH and thelower reference temperature value RTL, system 10 commands neither heaternor cooling. If the measured temperature falls to the level of RTL-H,heating is initiated by system 10.

FIG. 3D illustrates that heating is initiated if the measuredtemperature is less than RTL-A upon selection of the Automatic mode.When the heating sufficiently raises the temperature that MT is greaterthan or equal to RTL-A, system 10 signals heating system 30 to turn off.Thereafter, whenever the measured temperature becomes less than or equalto RTL-H, the heat is signaled on. If the temperature should rise sothat it is greater than or equal to RTH+H, the system changes over tocooling.

FIG. 3E illustrates an alternate embodiment of the Automatic mode, inwhich there is only one reference temperature value, RT, per timeinterval rather than a higher value RTH and a lower value RTL. Thisrequires only twenty-four reference temperature values for twenty-fourcontrol intervals. The operation in FIG. 3E is like that in FIG. 3C,with RTH=RTL=RT.

The Manual Mode illustrated in FIG. 3F is like that of FIG. 3E, exceptthat there is only one programmable reference temperature value, whichis the same for all the control time intervals. The operation shown islike that of FIG. 3D, with RTH=RTL=RT, with the same value of RT allday, until reprogrammed.

System 10 allows a user to adjust the thermostat setting for anindefinite period in any operating mode without altering the storedprofile of reference temperatures. At any time selected temperatureadjustment values ΔT, positive or negative, may be entered by means ofthe data entry switches 14 and a ΔT function switch 15. These adjustmentvalues are stored by unit 11 in memory 13 locations allocated to ΔT. Apreferred method of utilizing ΔT is as follows. When the controlfunctions such as RT+H are computed, the computation used would actuallybe (RT+ΔT)+H. Thus, for so long as ΔT is not equal to zero, the currentreference temperature is essentially modified by the amount of ΔT. Atypical situation in which this feature would be useful is where ΔT isgiven some non-zero value for a few hours to adjust the room temperaturefor comfort, then ΔT is reset to zero. In this case, it is onlynecessary to enter a value at the beginning of the adjustment period andregardless of the number of hours the adjustment is in effect. There isno requirement to remember the values of the reference temperatureprofile that is desired in the long run; these remain unchanged. Thecurrent value of ΔT is shown by display 18.

MULTI-STAGE OPERATION

System 10 provides for multiple stage operation of both heating andcooling systems. FIG. 4 depicts operation of the system in the Heatingmode of operation; however, the basic operating principle described hereis also applicable to other modes of operation. FIG. 4 depicts system 10controlling a heating system designed to be operated at two levels ofheat capacity, 66% and 100% of maximum. The operation of system 10 isthe same as that previously described herein, for the Heating mode ofoperation, and particularly as shown in FIG. 3, except that thecomparisons of current reference temperature value, RT and measuredtemperature value, MT include an additional parameter N. When MT is lessthan RT-N, where N is a programmable value, typically 2°, stored in theRAM 13 or ROM 12, unit 11 outputs a digital code to the output controlcircuit 26, that causes the heating system 30 to operate at fullcapacity. If MT is greater than RT-N, unit 11 causes the output controlcircuit 26 to output a control signal to cause the heating system 30 tooperate at its lower heat capacity.

FIG. 5 depicts one configuration of the output circuit 26 of system 10that can be utilized in a multi-stage application. There are circuits 43having discrete outputs for use with on/off, switch type controls insystems 28 and 30. Digital-to-analog converters 42 provide a signal forproportional control elements in systems 28 and 30. Signal conditioners44 and 48 typically have buffer amplifiers, line drivers, triacs and/orrelays interfacing converters 42 and circuits 43 with the various typesof controls used by systems 28 and 30.

The discrete circuits 43 respond to digital numbers and/or bits fromunit 11 and furnish latched outputs. They consist of latches (flipflops) and other logic required to interface with the signalconditioners. For the two stage heating mode depicted in FIG. 4, twodiscrete outputs interfaced with a two stage heating system, havingdiscrete type controls, will cause the system to operate at either 66%or 100% of its maximum capacity.

The digital-to-analog converters 42 respond to digital numbers from unit11, latch the data at the input and provide a steady state analogoutput, proportional to the value of the current digital input. For theheating mode depicted in FIG. 4, two analog levels when interfaced witha two stage heating system, having analog type variable controls willcause the system to operate at either 66% or 100% of its maximumcapacity.

Additional levels of control for systems having more levels of heatingand cooling capacity are provided by assigning more values of N,determining the magnitude of the output levels, sorting this data in theROM 12 or RAM 13, performing additional comparisons, as discussed aboveand outputting data to control circuit 26, to cause it to issue thenecessary outputs.

Additionally, the analog outputs of output circut 26 when applied tovariable controls such as variable flow rate valves can be related inunit 11 to the difference between the measured temperature and thecurrent reference temperature, thus providing proportional control.Alternatively, more complex linear or nonlinear control functions of themeasured temperature and the current reference temperature can beimplemented with the configuration illustrated in FIG. 5.

ZONE CONTROL

FIG. 6 and 7 illustrate the use of system 10 with the remote input andoutput of data in the control and regulation of temperature in differentareas or zones. In this application, system 10 enables the temperaturein each area to vary independently, according to a programmed sequenceof reference temperatures, as discussed previously. Additionally, thezone requiring the most heating or cooling determines the operatingcapacity level of the heating and cooling systems as described withrespect to multi-staging systems.

FIG. 6 shows one method that provides zone temperature control andregulation. The zones are depicted as separate areas; however, each zonecan be either separate or combined with other zones. Each zone issupplied with a hot deck 63 and a cold deck 64 having variable positiondampers 61 and 62, respectively. Reversible positioning devices 65 aremechanically attached to dampers 61 and 62, thereby causing them toreversibly open or close in response to control signals from system 10.

The temperature in each area is therefore determined by the temperaturesof the hot and cold decks 63 and 64 and by the positions of the dampers61 and 62 which mix the air exiting the hot and cold decks, in thecorrect proportion. The position of dampers 61 and 62 are determined bysystem 10 from temperature transducer 97, remotely located from system10, in each zone. Circulation in the zones is provided by the fan 67forcing air through the heating and cooling systems 30 and 28, throughthe hot and cold decks 63 and 64, dampers 61 and 62, and back throughthe return air duct 66, and damper 99 to the fan.

Temperature transducers 72 and 73 in the hot and cold decks 63 and 64are used by system 10 to adjust the operating capacity levels of thesystems 30 and 28, to ensure adequate hot and cold deck temperatures,and to monitor system operation and detect system malfunction.Temperature transducer 69 and 75 in the return and external air ducts 66and 100 are used by system 10 to reversibly control damper positioners98 and 68 which operate dampers 99 and 74 in the return air duct andexternal air duct to input fresh air into the zones.

A display and control panel 71 in each zone, or at a control point,remotely located from system 10 provides for display and entry of timeintervals, time of day, temperature, etc.

FIG. 7 shows the configuration of the data input/output section ofsystem 10 for this application. The display/data entry panels 27 locatedin the zones or at a remotely located control point enable the timeintervals and corresponding reference temperatures for each zone to beentered into the RAM 13 via unit 11. These values can be the same forall zones or they can be unique to each zone. If they are unique for oneor more zones, system 10 stores them in the RAM 13 which is partitioned,or the data is otherwise identified, such that it can always be relatedto the particular zone to which it corresponds. The data transfer to andfrom the remote display/data entry panels is accomplished via anextension of the system 10 control and data bus to each zone, to providefor either a parallel or serial data flow on either a preprogrammed,priority or sequential basis.

The outputs of the temperature transducers 20, located in the zones, inthe return air duct and in the external environment is fed to the signalconditioners 22. The signal conditioners buffer and scale the signals tothe proper value and feed them to the multiplexer 25. Under control ofunit 11, the signal is fed through the multiplexer 25 to theanalog-to-digital converter 24, where it is converted to a digitalnumber. The measured temperature for each zone is determined in a likemanner by unit 11, on a preprogrammed, priority or sequential basis.Each temperature value is then compared to the control function asdetermined from the correct reference temperature, hysteresis andanticipator values, for each zone as previously described andparticularly as shown in FIGS. 3 and 4.

Additionally, the external temperature measurement can be utilized bysystem 10 in the computation of the control functions and a positive ornegative external temperature adjustment XΔT can be made to the currentreference temperature value. For example, in using the control functionRT+H, the actual computation would be (RT+X ΔT) +H. One approach tocomputing a value for XΔT is to multiply the differences between theexternal temperature and the current reference temperature by a positiveor negative factor, the value of which has been programmed into system10 by means of the data entry and functions switches 14 and 15. Anotherapproach is to provide for a set of XΔT values to be entered viaswitches 14 and 15, each value of XΔT to be used in a particular rangeof the external temperature.

The availability of programmable reference temperatures for twenty-four,one-hour intervals will be satisfactory for many applications of system10. In those cases where it is desired to vary the reference temperaturemore smoothly, it is not absolutely necessary to provide moreprogrammable reference temperature values. The microprocessor can beprogrammed to use the stored reference temperature values at the ends ofthe time intervals, but to calculate a reference temperature valuewithin the interval, by interpolation.

System 10 is interfaced to the heating and cooling systems 28 and 30,damper positioners 5, etc., through the output control circuit 26. Itcontains discrete on/off type circuits 43, digital type circuits 45,digital-to-analog converters 42 and signal conditioners 44. The discretecircuits 43 respond to digital numbers and/or bits from unit 11 andprovide latched and/or unlatched outputs. The digital type circuits 45respond to digital inputs from unit 11 and provide parallel and/orserial type outputs. The digital-to-analog converters 42 respond todigital inputs from unit 11, latch the data at the input and provide asteady state analog output proportional to the value of the currentdigital input. The signal conditioners 44 consist of buffer amplifiers,line drivers, triacs and/or relays as may be required to interfacesystem 10 to the various types of damper positioners and controls usedby systems 30 and 28.

In each of the above embodiments, it is preferable that output circuit26 include means for ensuring that the output remains in one state for aselected minimum time before being switched to another state. Thisprevents system 28 or 30 from being switched off, then immediately backon, for example.

FIG. 8 illustrates one of several possible alternate means forconverting the temperature measured by transducer 20 from an analog formto a digital number. Unit 11 outputs a series of digital numbers to thedigital-to-analog converter 32 such that a successive approximation typeconversion can be made. The converter produces an analog outputproportional to the value of each digital input. The analog output, foreach digital input, is then compared in the analog comparator 34 to theanalog input from the temperature transducer 20, via the signalconditioner 22. Thus, a comparison is made for each successive digitaloutput from unit 11. This process continues, until comparator 34 sensesthat the analog values are equal or within the range of one leastsignificant bit. When this occurs, comparator 34 sends a signal 36 tounit 11 to stop the process. The last digital output from unit 11 thusrepresents the analog value of the measured temperaure from transducer20.

The capability for obtaining remote input/output with system 10 allowsit to carry on important performance monitoring functions. For example,system 10 can detect when systems 28 or 30 fail in their heating orcooling functions. When unit 11 issues a turn on signal for one of thesystems 28 or 30, it can store in memory 13 a record of the currenttemperature and time. Subsequently, when it brings in new digital valuesof the measured temperature, it can make the determination as to whetherthe heating or cooling system has changed the measured temperatureappropriately, considering the time elapsed since turn on. If themeasured temperature has failed to respond properly, then system 10 cansound an alarm 40 and/or show a malfunction indication on display 18.Similarly, discrete on/off type temperature sensitive switches locatedin various elements of the heating and cooling systems can be used bysystem 10 to detect malfunction or unsafe conditions. These conditionscan also be detected from temperatures measured by analog transducersand compared in unit 11 to stored limit values.

System 10 is normally operated by the electrical power system used forcooling and heating systems 28 and 30. However, means are provided forautomatically switching system 10 to battery operation when theelectrical power system fails. In a preferred embodiment, system 10includes means for maintaining the battery charged from the electricalpower system, when it is operative. The battery backup operation isnecessary to prevent system 10 from losing time of day or referencetemperature values that may be stored in volatile memory, during poweroutages.

The exhibition of a number of the variables by display 18 have beendescribed above. These and others that are usefully displayed indicatedin FIG. 1. They include the measured temperature, an indication ofwhether heating or cooling is presently commanded, the number entered onswitches 14, the present time of day, the current reference temperature(including higher and lower values), temperature adjustments (ΔT andXΔT), the heating/cooling malfunction indication, and the duration ofthe control intervals (typically one hour).

IMPROVED SYSTEM AND METHOD

A number of additions and modifications are possible for the describedpreferred embodiments. Several of the possible improvements which may bemade are described below.

As pointed out above, one use of system 10 is to provide temperaturecontrol for a person who is away from home at work during the day. Forsuch a person, the daily profile of reference tempratures desired for awork day is different from the profile desired for a day off work.Accordingly, it is important to provide for at least one additional setof reference temperature values to be used on off days. The day offreference values are stored in random access memory 13 just as the workday values are.

Where system 10 is to use two different sets of reference temperaturesfor different days, it can be manually switched to the alternate set, orarithmetic and control unit 11 can note what day it is, as it is keepingtrack of the time hour by hour. Then, at the beginning of each new day,the control program for the system checks on which set of referencevalues are to be used that day and provides that the referencetemperature values used are taken from the appropriate memory locations.

An additional set of locations in memory 13 contain a code for each day,indicating which set of reference values are to be used on that day. Forthe typical user, it is satisfactory to provide seven memory locations,each corresponding to a day of the week, with control unit 11automatically cycling through the seven locations each week. The codesfor each day of the week are entered by using function switches 15 anddata entry switches 14.

For users with more complicated schedules, more memory locations can beprovided, one for each day over a period of several weeks or a month.

It will often be desirable to delay the onset of a new referencetemperature. For example if one is staying up an hour longer than usualat night, it is convenient to delay the onset of a programmed sleepingperiod reference temperature for an hour. One way this is accomplishedis with a delay value that can be entered, specifying that the currentreference temperature is to be held for the entered period. In thisembodiment, the program sequence of system 10 includes steps which checkto see if a delay has been entered and whether the entered delay periodhas expired. Only when the entered delay period has expired is thesystem free to release the old reference temperature and use the oneordinarily appropriate to the current time.

Another way of delaying the onset of a new reference temperature is toswitch the system 10 into the Manual Mode until it is desired for thenew reference temperature to become effective. As described hereinabove,system 10 is designed so that there is a reference temperature valuespecifically for the Manual Mode. A possible variation in design is tohave the Manual Mode use as its reference temperature, the referencetemperature value in effect when the system is switched into ManualMode. This is, of course, quite useful in manually delaying the onset ofa new reference temperature.

In an alternate embodiment of the data entry previously described,system 10 provides for the automatic entry of reference temperaturevalues for consecutive time periods in those cases where the values arethe same. First, the selected reference temperature value is entered forthe first time period for which it is to be used. Then, the usercontinues to actuate a switch which causes system 10 to enter this firstvalue for succeeding time periods, until the time period is reached, forwhich a different reference temperature value is to be entered. In thismanner, data entry is simplified, since it is not necessary to enterreference temperature values for every time period, unless each value isdifferent from the previous one.

Under the constraints of current technology, system 10 is preferablypowered from the AC line in a building, and can run only a short timefrom a small battery. In one embodiment, system 10 responds to aninterruption of the AC power by shutting itself down except for randomaccess memory 13, control unit 11 and other portions of the systemrequired to keep accurate time. Those portions not shut down are poweredby a battery. At the end of the AC power outage, system 10 is progammedto test its battery voltage with a comparator circuit or usinganalog-to-digital converter 24. If the battery voltage is above apredetermined level, it is supposed that the battery has adequatelysupplied power to the portions of the system it supported during the ACoutage. In this case, system 10 continues its operation as though notinterrupted. If the battery voltage is too low, stored values ofreference temperatures and time may be incorrect. In the latter case,system 10 adopts preselected nominal values of reference temperaturesand gives an indication for the user to reset time and referencetemperature values.

One other test the system 10 makes to determine whether or not referencetemperature values stored in the random access memory 13 has changed, isto perform an addition of the reference values when they are initiallystored or changed by the user. The sum of these values is then stored inthe random access memory 13. Thereafter, on a periodic basis andparticularly after every power outage, system 10 immediately performs anaddition of the values again and compares this sum with the value of thepreviously computed sum stored in the random access memory 13. If theycompare, system 10 continues normal operation. If they do not compare,system 10 adopts preselected nominal values and gives an indication forthe user to reset the reference temperature values.

In an improved embodiment of the control program for system 10, there isan additional check point before issuing a command to output circuit 26.Each time a command is given to turn off cooling system 28 or heatingsystem 30, the program begins to keep track of the time elapsed afterthe turn off command. Then, before it issues a turn on command to thesame system 28 or 30, it checks to determine that a preselected time haselapsed since the turn off command. This prevents turning on one of thesystems 28 or 30 too soon after it has been turned off, and isparticularly important with respect to air conditioning units.

System 10 has a capability considerably expanded beyond conventionalsystems for monitoring its own operational variables. As mentionedabove, it can by use of its temperature sensors and other remotetransducers, detect unsafe or undesirable temperature conditions or someoverall malfunction. In addition, other variables, such as the overallcurrent drawn by the system can be monitored to produce alarms or modifythe response of the system accordingly.

Although preferred embodiments of the invention have been described indetail, it is to be understood that various changes, substitutions, andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A thermostat for generating control signals for atemperature modifying device comprising:(a) means for generating adigital electrical signal representative of the ambient temperature onthe thermostat; (b) manually accessible means for storing a plurality oftime/temperature schedules in a programmable digital memory, saidschedules providing a digital electrical signal representative ofdesired temperatures for different times within the interval defined byeach schedule; (c) means for generating a digital electrical signalrepresentative of time; (d) comparator means having first and secondinputs with one input coupled to said ambient temperature generatormeans; (e) interface means coupled to said digital time generating meansand operative to selectively one of the schedules to the second input ofsaid comparator means; and (f) means for coupling the output of saidcomparator means to said temperature modifying device.
 2. The system ofclaim 1, wherein said time is a time of day value generatedelectronically in digital form.
 3. The system of claim 2, furtherincluding means for electronically displaying said time of day value. 4.The system of claim 1, wherein:(a) said time schedules are identified bystoring the desired time of day that an interval is to begin, and (b)wherein the time stored to indicate the beginning of a second andsuccessive scheduled interval also indicates the end of the priorscheduled interval.
 5. The system of claim 1, wherein said comparatormeans includes:(a) means for generating a signal capable of initiatingheating when the ambient temperature is less than the desiredtemperature value minus a hysteresis value; and (b) means for generatinga signal capable of terminating heating, when the ambient temperaturebecomes substantially equal to the desired temperature value.
 6. Thesystem of claim 1, wherein said comparator means includes:(a) means forgenerating a signal capable of initiating cooling when the ambienttemperature is greater than the desired temperature value plus thehysteresis value; and (b) means for generating a signal capable ofterminating cooling when the ambient temperature decreases to thedesired temperature value.
 7. The system of claim 1, wherein:(a) saidscheduled temperatures, including said current scheduled temperature,are relatively higher temperatures; (b) further wherein said manuallyaccessible means enters a sequence of digital numbers, each representinga value of a relatively lower scheduled temperature and eachcorresponding to one of said time schedules; (c) means, accessible to auser, for entering and storing a digital code indicative of a selectedmode of operation; (d) control means for identifying the current one ofsaid lower scheduled temperatures corresponding to said current timescheduled and for storing a digital number representing the value ofsaid identified lower scheduled temperature; (e) wherein said controlmeans further includes means for generating, in response to a firstvalue of said digital code, signals for controlling heating; (f) meansfor generating, in response to a second value of said code, signals forcontrolling cooling; (g) means for generating, in response to a thirdvalue of said code, signals for controlling heating in accordance withsaid current lower scheduled temperature value and cooling in accordancewith said current higher scheduled temperature value, and (h) means forgenerating, in response to a fourth value of said code, signals forcontrolling both heating and cooling in accordance with the same one ofsaid scheduled temperature values during all of said intervals.
 8. Thethermostat of claim 1, wherein said schedules provide a listing ofdesired temperatures throughout a 24-hour period and wherein saiddigital time generating means provides a 24-hour period signal to saidinterface means wherein said interface means selects one of theschedules for the particular 24-hour period.
 9. The thermostat of claim8, wherein said interface means further includes means for coupling thedesired temperature from the selected schedule to said other comparatorinput according to a time of day signal supplied by said digital timegenerating means.
 10. The thermostat of claim 1 for controlling theapplication of electrical power to a singular electrically controlledtemperature modifying device comprising:(a) a programmable, digitalmemory; (b) means for loading said memory through said manuallyaccessible means with a plurality of separate schedules for alternativeuse in controlling the singular temperature modifying device, eachschedule comprising digital signals representing desired temperatures atparticular times over a twenty-four period; (c) means for selecting oneof said plurality of schedules; (d) circuitry for applying the output ofthe time generating means to the memory to generate a digital electricsignal representative of the desired temperature at a particular time asstored in said selected schedule; (e) said coupling means receiving theelectrical signal having a characteristic which is a function of ambienttemperature and the output of the memory and generating a control signalto the singular temperature modifying device as a function of theirdifference; and (f) so that said interface means for selecting one ofsaid schedules is controlled in accordance with different days of theweek.
 11. The thermostat of claim 10 including a display device andmeans for connecting the display device to ambient temperature and tosaid time generating means.
 12. The thermostat of claim 10 wherein saidtime generating means comprises a digital divider chain driven by aperiodically varying electrical signal having a frequency independent ofthe ambient temperature of the device.
 13. The thermostat of claim 10wherein the singular temperature modifying device is a furnace.
 14. Thethermostat of claim 10 wherein the singular temperature modifying deviceis an air conditioner.
 15. The thermostat of claim 10 wherein theambient temperature, generating means in the thermostat is digital. 16.The thermostat of claim 1 wherein components of the thermostat comprisean integrated semi conductor circuit.
 17. The thermostat of claim 1 forgenerating control signals for a singular temperature modifying device,comprising:(a) said time generating means comprising a clock forgenerating a digital electrical representative of the time-of-day andday-of-week; (b) a programmable digital memory; (c) said manuallyaccessible means loading said memory with a plurality of separateschedules for alternative use in controlling the singular temperaturemodifying device, each schedule comprising digital signals representingdesired temperature at particular times over a one day cycle; (d) meanscontrolled by the day-of-week signal for selecting one of said pluralityof schedules; (e) circuit means for interrogating said memory with saiddigital time-of-day signal to generate a digital signal representativeon the desired temperature at the existing time as stored in saidselected schedule; and (f) wherein said comparator is operative toreceive the ambient temperature on the thermostat and the signalrepresentative of the desired temperature from said circuit means, andto provide from said coupling means a control signal to the singulartemperature modifying device to maintain the ambient temperature at saiddesired temperature.
 18. The thermostat of claim 17 wherein said meansfor loading said memory with a schedule of desired temperatures fordifferent times over a one day cycle includes a manually operablekeyboard on said thermostat.
 19. The thermostat of claim 17 includingdigital means for modifying the output of the memory to generate asignal which is provided to the comparator means to offset thetemperature occurrence of the control signal with respect to the ambienttemperature.
 20. The thermostat of claim 17 wherein the singulartemperature modifying device is a furnace.
 21. The thermostat of claim17 wherein the singular temperature modifying device is an airconditioner.
 22. The thermostat of claim 17 wherein said schedulesprovide a listing of desired temperatures throughout a 24-hour periodand wherein said time generating means provides a 24-hour period signalto said interface means and said interface means selects one of theschedules for the particular 24-hour period.
 23. The thermostat of claim17 wherein said interface means further includes means for coupling thedesired temperature at a given time from the selected schedule to saidsecond comparator means input according to a time of day signal suppliedby said time generating means.
 24. A thermostat for controlling theapplication of electrical power to a temperature modifying load,comprising:(a) means for generating an electrical signal having acharacteristic which varies as a function of the ambient temperature onthe thermostat; (b) a clock operative to generate digital electricalsignals having values representative of real time; (c) a programmable,digital memory; (d) means for loading said memory with a plurality ofseparate programs each comprising digital signals representing desiredtemperature at particular times over a repetitive time cycle; (e) meansfor selecting one of said plurality of programs; (f) circuitry forapplying the output of the clock to the memory to generate a digitalelectric signal representative of the desired temperature at aparticular time as stored in said selected program; (g) means forreceiving the electrical signal having a characteristic which is afunction of ambient temperature and the output of the memory and forgenerating a control signal for said temperature modifying devices as afunction of their difference.
 25. The system of claim 24, includingmeans for visually displaying said clock generated time values.
 26. Thesystem of claim 24, further including means for displaying temperaturefrom said temperature generating means.
 27. The system of claim 26,wherein the desired reference temperature values are stored inelectronically readable form.
 28. The system of claim 24, furtherincluding:(a) means for storing digital numbers representative of ananticipator and hysteresis values and wherein said means for receivingand for generating a control signal is capable of activating thermalmeans dependent on the relation of the ambient temperature to thedesired temperature as modified by the hysteresis value; and (b) saidmeans for generating a control signal is also capable of deactivatingsaid thermal means, dependent on the relation of the ambient temperatureto the desired temperature value as modified by the anticipator value.29. The system of claim 24, wherein said time of day is generated indigital form.
 30. The system of claim 29, further including means forelectronically displaying said time of day value.
 31. The system ofclaim 29, further including means for electronically displaying saidmeasured temperature.
 32. The system of claim 29, wherein(a) said timeintervals are identified by storing the desired time of day that aninterval is to begin; and (b) wherein said time of day stored toindicate the begin g of a second successive interval also indicates theend of the prior interval.
 33. The system of claim 29, including meansfor electronically displaying said time interval values.
 34. The systemof claim 29, further including memory means for storing the referencetemperature values in electronically readable form in memory.
 35. Thesystem of claim 29, wherein said signal representation of ambienttemperature is an analog signal and said receiving means include meansfor converting said analog signal to a digital signal.
 36. Thethermostat of claim 24, including a display device and means forconnecting the display device to said electrical signal having acharacteristic which varies as a function of ambient temperature and tothe output of the clock.
 37. The thermostat of claim 24, includingmanually operable means for programming said memory.
 38. The thermostatof claim 24, wherein said clock comprises a digital divider chain drivenby a periodically varying electrical signal having a frequencyindependent of the ambient temperature of the device.
 39. The thermostatof claim 38, wherein said generating means includes means for receivingsaid periodically varying signal and said electrical signal having acharacteristic which varies as a function of the ambient temperature onthe thermostat.
 40. The thermostat of claim 24, wherein the electricalsignal having a characteristic which varies as a function of the ambienttemperature in the thermostat is a digital signal.
 41. The thermostat ofclaim 40, wherein components of the thermostat comprise an integratedsemi conductor circuit.
 42. The thermostat of claim 24, including asource of a constant frequency, periodically alternating electricalsignal forming part of both said means for generating an electricalsignal having a characteristic which varies as a function of the ambienttemperature on the thermostat and said clock.
 43. The thermostat forgenerating control signals for a temperature modifying device,comprising:(a) means for generating a digital electrical signalrepresentative of the ambient temperature on the thermostat; (b) meansfor generating a digital electrical signal representative of the timewithin a repetitive time cycle; (c) a programmable digital memory; (d)means for loading said memory with a plurality of separate programs eachcomprising digital signals representing desired temperature atparticular times over a repetitive time cycle; (e) means for selectingone of said plurality of programs; (f) circuit means for interrogatingsaid memory with said digital time signal to generate a digital signalrepresentative of the desired temperature at the existing time as storedin said selected program; and (g) a comparator operative to receive thedigital signal representative of the ambient temperature on thethermostat and the digital signal representative of the desiredtemperature, and to provide a control signal to said temperaturemodifying device.
 44. The system of claim 43, further includingelectronic memory means for storing said desired temperature values. 45.The system of claim 43, wherein said comparator means includes:(a) meansfor generating a signal for initiating heating when the ambienttemperature is less than the desired temperature value minus thehysteresis value; (b) means for generating a signal for terminatingheating when the ambient temperature increases to the desiredtemperature value; (c) means for generating a signal for initiatingcooling when the ambient temperature is greater than the desiredtemperature value plus the hysteresis value; and (d) means forgenerating a signal for terminating cooling when the ambient temperaturedecreases to the desired temperature value.
 46. The system of claim 43,further including:(a) means for storing a digital number representativeof a change to be produced in the ambient temperature by saidtemperature modifying device in response to said control signals) and(b) means responsive to the ambient temperature signal representationand to said control signals and to said number representative of achange for generating an alarm indication if said change is not actuallyproduced in response to said control signals.
 47. The system of claim43, further including:(a) means accessible to a user for entering adigital number representing an ambient temperature adjustment; (b) meansfor repeatedly generating a digital number representative of the presentvalue of an external temperature; and (c) wherein said comparatorincludes means for in response to said external temperature number,generating said control signals with the ambient temperature valuemodified by the value of said adjustment number.
 48. The system of claim43, wherein said control signals include an analog of a linear functionof the measured temperature and the current reference temperature. 49.The thermostat of claim 43, wherein said generating means comprises anoscillator which has a constant frequency output at ambient temperaturesover the range of the thermostat and a divider chain operative toreceive the output of the oscillator.
 50. The thermostat of claim 43,wherein said means for loading said memory with a program of desiredtemperatures for different times over a repetitive time cycle includes amanually operable keyboard.
 51. The thermostat of claim 43, includingdigital means for modifying the output of the memory to generate asignal which is provided to the comparator in order to offset thetemperature occurrence of the control signal with respect to the ambienttemperature.
 52. A thermostat for generating control signals for atemperature modifying device, comprising:(a) means for generating adigital electrical signal representative of the ambient temperature onthe thermostat; (b) means for generating a digital electrical signalrepresentative of the time within a repetitive time cycle; (c) a digitalmemory for receiving and storing entries thereinto; (d) manuallyoperable, data entry switch means for inputting a plurality oftime/temperature schedules wherein each of said time/temperatureschedules provides a digital representation of schedule temperature atdifferent times in two or more intervals within each of said schedules;(e) microprocessor means responsive to the signal representative of timefor periodically comparing ambient temperature represented by thedigital signal from said means for generating and the scheduledtemperature for the scheduled time interval for the particular schedule,said microprocessor means forming an output signal resulting from saidcomparison; and (f) output circuit means connected to saidmicroprocessor means for receiving the output signal thereof forming acontrol signal for said temperature modifying device to controloperation thereof and modify the ambient temperature at the thermostat.53. The thermostat of claim 52 wherein said switch means enters two ormore time/temperature schedules, each of said schedules being for 24hours and representative of weekday and weekend time periods, andfurther including mode switch means enabling AM/PM time schedule entry.54. The thermostat of claim 52 wherein said output circuit heating andcooling means comprises separate control means, each of which includesmeans connecting to heating and cooling systems.